stdlib/String: implement Unicode extended grapheme cluster segmentation

algorithm

The implementation uses a specialized trie that has not been tuned to the table
data.  I tried guessing parameter values that should work well, but did not do
any performance measurements.

There is no efficient way to initialize arrays with static data in Swift.  The
required tables are being generated as C++ code in the runtime library.

rdar://16013860


Swift SVN r19340

stdlib/core/StringUnicodeScalarView.swift

@@ -62,6 +62,16 @@ extension String {
         return IndexType(i, _core)
       }
 
+      /// The end index that for this view.
+      @internal var _viewStartIndex: IndexType {
+        return IndexType(_core.startIndex, _core)
+      }
+
+      /// The end index that for this view.
+      @internal var _viewEndIndex: IndexType {
+        return IndexType(_core.endIndex, _core)
+      }
+
       var _position: Int
       var _core: _StringCore
     }